America’s lust for gigantic breasts leads to impotence: the population genetics of captive-reared turkeys

Gobble? image from http://www.public-domain-image.com

The noble turkey, a centerpiece of the American Thanksgiving supper. It looms large from its prominent position on the dining room table. The master of ceremonies – or, in my case, the guy who keeps slicing himself open with various sharp objects yet is inexplicably the one people call on when there’s knife-work needs doing – draws a set of fine, honed knives, set aside for this particular task, and carves, delicately yet firmly, into the hefty white meat of the turkey’s breast. Sure, some favor the dark, rich meat around the legs, but this white meat, soaked in gravy and topped with cranberry sauce or stuffing, that is what we crave.

“We give thanks,” the benediction may begin, “to Charles Darwin, for determining the underlying mechanism by which a theropod may, over the course of 65 million years, through a process of gradual change by means of the retention of beneficial traits through successive generations, evolve into this delicious, delicious bird.” And then, perhaps, that surly teenager, the one determined to point out the social inequalities inherent in the holiday and the colonialist attitudes which led to the wholesale extermination of America’s native peoples – every family has at least one – will chime in to quip “you know, evolution didn’t shape the turkey. The modern Thanksgiving turkey is the product of an extensive selective breeding program that began in the 1940’s. Commercial turkeys can’t even reproduce naturally, they have to be artificially inseminated.” At which point the older members of your family may blush and/or faint at such an unseemly turn of phrase.

The problem with teenagers is that they’re often right in the most obnoxious way possible.

The modern commercial turkey is a triumph of selective breeding over the limitations imposed by physiology. This is what turkey mating looks like in the wild:

Pay attention to the moment where the hen’s (the female on the bottom) tail raises up vertically and the tom (the male on top) lowers down to touch vents. This is the point where sperm transfer occurs, the cloacal kiss.

Commercial turkeys are not so lucky. The giant breasts we enjoy so much are too large to allow a tom to effectively mount a hen. In addition to unnaturally inflated breasts, broad-breasted whites (the most common breed of commercial turkey) are bred to efficiently convert feed to meat, at a ratio of 2 to 1. They add mass quickly, reaching market weight in 16 weeks. This rapid weight gain results in a bird that, by the time it reaches sexual maturity, has difficulty supporting its own weight; their legs are often not strong enough.

The result of this is that nearly 100% of commercial domestic turkeys are the product of artificial insemination. A relatively small breeding stock is used to maintain the entire commercial turkey industry. Breeding turkeys are tougher and slower growing than their broiler offspring (although an older adult breeding tom can weigh more than 60 pounds), so the odds are that the turkey on your plate probably hasn’t reproduced, even by proxy. This reduced breeding population results in a genetic bottleneck. Genetic diversity among domestic turkeys is low. The resultant inbreeding depression adversely affects the immune system, increasing disease susceptibility, and creating a greater demand for antibiotics.

This is not the first time domestic turkeys have undergone a genetic bottleneck. There is evidence that turkeys were domesticated in Mesoamerica as early as 800 B.C. Genetic studies of pre-colonial turkey remains suggest that wild turkeys were domesticated at least twice prior to European settlement. A major population bottleneck was detected in a southwestern wild turkey lineage that is consistent with domestication and transportation over almost 1000 years. This was a turkey with a long history of domestication and cohabitation with native southwestern tribes.

One way to escape inbreeding depression is to introduce genetic variability from wild and heritage turkey populations. Heritage breeds are domestic turkeys that retain some ancestral characteristics. Heritage populations of all livestock have plummeted in the last century, as commercial producers favor a few, highly profitable breeds. Thus, heritage breeds are maintained largely by “turkey fanciers”, people who raise specific breeds for fun, to show, or as high-priced alternatives to industrial farmed turkeys (for the record, I have four heritage breed chickens and Amy has a pair of heritage goats). But crossing commercial and heritage turkeys is not a guarantee of success, some heritage breeds are only distantly related to modern commercial breeds, and inbreeding depression remains a problem. Maintaining reasonable levels of genetic diversity in commercial domestic turkeys is likely to remain a challenge for the foreseeable future.

It’s not all bad news from the commercial turkey industry. The process of artificially inseminating birds, developed and perfected in the commercial industry, has been a boon to avian conservation. With 1,257 bird species listed as at least vulnerable on the IUCN Red List, 4 of which are extinct in the wild and now only exist in captivity, captive breeding programs provide one effective way to restock wild populations. Artificial insemination programs have been essential in the re-population and de-listing (or downgrading) or Peregrine falcons, California condors, and Whooping crane. These programs owe part of their success to the innovation and experience of the commercial turkey industry.

If you’re inclined to avoid commercial domestic turkeys on your dinner plate, you can seek out heritage breeds or wild turkey. Both of which are very expensive options (unless you choose to hunt wild turkey) and will put much less meat on your dinner table. Organic and free-range are still from the same breeding stock, so that won’t make any difference in the issues above (and really, if a free-range broiler can barely walk, it’s not going to be roaming among the heather anyway). If you’re interested in taking up the plow (so to speak) you can try your hand at raising your own Thanksgiving turkey. There a several suppliers that will sell you day old heritage poults which you can raise to maturity. Not surprisingly, I tend to prefer that option, as you’re guaranteed that your dinner had a decent life and it’s a great opportunity to teach your children where their food comes from. Just be aware that you may end up with a charismatic and unusual pet.